1. Field of the Invention
The present invention relates to a novel insulated coil. More particularly, it relates to an insulated coil for use in high voltage rotary electric machines such as turbine generator and water wheel generator.
2. Description of the Prior Art
Recently, the need has arisen for an insulated coil which exhibits highly stable insulating properties under severe operating conditions such as repeated starting and stopping of heat cycles, the thermal deterioration caused by rising temperatures and the shortcircuiting caused by mechanical vibration,
The conventional insulated coil for high voltage rotary machine has been proposed by coating a conductor with a suitable insulating layer, shaping it into a predetermined shape, winding a mica tape made of a back support, of a fibrous support a nonwoven support or a film support coated with integrated mica or split mica about the conductor impregnating a thermosetting resin such as unsaturated polyester resin or an epoxy resin under vacuum or vacuum-pressure conditions and heat-molding it.
These insulated coils are assembled in a rotary machine and are usually used in a gas environment. Under high voltage, corona discharge is caused by breakdown of the gas such as air in the coil and it may cause breakdown of an insulating layer. In order to prevent the trouble, mica has been used in the coils for high voltage rotary machine. Although excellent insulating materials have been developed, mica has been used as an important insulating material for insulation of a coil because of its excellent heat resistance, corona resistance and voltage breakdown resistance characteristics. Mica is classified into split mica and integrated mica. The mica is bonded on a back support made of a paper, a film, or a porous insulating material such as glass fabric, or a polyester nonwoven fabric with a thermosetting resin to prepare a mica tape or sheet. An integrated mica tape insulating layer and a split mica tape insulating layer are each prepared by impregnating a mica tape or sheet in vacuum or vacuum-pressure impregnating with a thermosetting resin and curing the resin and are tested in comparison. The split mica tape insulating layer has a higher mechanical strength, for example, a higher bending strength of about 120 to 150%, but has lower electric property, for example, lower breakdown voltage as about 75-90% in comparison with those of the integrated mica tape insulating layer. Each mica tape insulating layer has advantages and disadvantages and accordingly, it is not suitable to use only one type.
The integrated mica layer of the conventional integrated mica tape is prepared by forming mica flakes into sheets and consequently the density of the mica flakes is low. In practice, slippage occurs between mica flakes during the taping operation of the integrated mica tape thus resulting in deterioration of the high breakdown voltage of the integrated mica.
The strength of the mica tape is depending upon the back support. Paper, a polyester nonwoven fabric and a film are relatively economical, but have relatively low tensile strength whereas a glass cloth has high tensile strength, but is expensive. Thus, a desired back support has not been found.